German published examined patent application No. 15 02 335 discloses a throttling device of the type as defined in which the transfer channels are in the form of a partial annular groove formed concentrically with the respective axes of the cylindrical bores, the two transfer channels being arranged fully overlapping one another in the axial direction. The retaining plates are in the form of kneading discs or double conical discs, it being possible for cylindrical sections to be formed in the latter in the transient area between the conical sections and an annular surface perpendicular to the axis, whereby a higher resistance to wear in the transient area between the annular surfaces and the double conical surfaces is to be achieved.
The disadvantage of this known solution is that a linear throttling characteristic is not achieved in the event of relative displacement between the housing and retaining plates. In the case of screw-type extruders generally, and particularly in the case of extruders for viscoplastic compounds which are used particularly for melting and homogenizing plastic materials, it is advantageous if the degree of melting, the melt pressure and the melt temperature can be set in specific process zones irrespective of the type of plastic material to be processed and the other operating conditions. Infinitely variable throttling devices which are subject to a great number of requirements are used in this case. With regard to process technology a correlation as linear as possible is to be achieved between the relative displacement of the retaining plates and housing and the melt pressure or pressure drop in the throttle. Furthermore, the throttle is not intended to act as an energy consumer when in the open state, i.e. the melt shall not be subjected to a pressure drop in the throttle. Finally, the throttle shall be operable from the fully open to the almost closed state. With regard to design and operating technique the throttling device shall not be susceptible to trouble during short- or long-term operation. It is intended to have in general a long service life and to be altogether very short with a simple construction. Numerous other throttling devices which are known for example from German published patent applications Nos. 29 24 269, 29 24 800 and German patent No. 26 50 742 cannot fulfil this requirement, that is particularly the requirement for a linear characteristic with, at the same time, a high operational reliability.